This invention relates to a method for evaluating a quality of an image on a display device, more particularly to a method for evaluating the image quality, especially granularity, of an image displayed on CRT (cathode ray tube), LCD (liquid crystal display) or other soft copy display devices (which are hereunder collectively referred to as “display device”).
Evaluations of the image quality of images on display devices such as the CRT, the LCD and the others, as exemplified by brightness, image resolution and granularity, provide important information when selecting the right display device to be used by comparing the performance of one display device with that of another. Image quality evaluation is also very important for the purpose of evaluating the degree of image deterioration that occurs during the use of a specific display device. Whichever type of evaluation is to be performed, it is essential that the quality of images on display devices be evaluated quantitatively.
Before shipping the display device they have produced, manufacturers test and adjust on their own to verify or check that the intended image quality is ensured. However, once the display devices have been shipped, no practical methods and devices are available for verifying image quality and it is the user alone who is responsible for checking the quality of the image being displayed. Most users rely upon visual evaluation to check the quality of the images being presented on display devices.
Visual evaluation is performed using test patterns capable of integrated verification of various image quality and characteristic factors and an SMPTE pattern is best known of such test patterns. Depending on the need, the user of a display device allows it to display a test pattern such as an SMPTE pattern, examines it, checks only superficial or very basic features of the quality of the image being shown on the display device, and determines whether its use should be continued or not.
In practice, what the user can say after having applied this visual approach is that “two devices produce images that look differently in this particular area” (comparison between two devices) or, speaking of a certain device, “it seems to produce an image of which a particular area looks slightly different than it did initially” (time-dependent change of the device); in other words, the visual method can only perform very subjective and qualitative evaluation and it has been impossible to achieve quantitative and objective evaluation.
In particular, the development of a specific method or device for enabling quantitative visual evaluation of granularity has been practically nil.